Field of the Invention
The present invention relates to an energy recovery apparatus which uses a water treatment system of the reverse osmosis method in order to desalinate seawater, or obtain clean water from polluted water.
Description of the Related Art
Reverse osmosis is known as one method for producing fresh water from seawater. This reverse osmosis method applies to seawater a pressure that is higher than the osmotic pressure of seawater (approximately 2.5 MPa) in a direction that is opposite to the direction of exertion of the osmotic pressure, conducts filtration with a semipermeable membrane (reverse osmosis membrane), and separates fresh water by removal of salts from the seawater. The seawater in which salts have been condensed and from which freshwater has been separated flows out from a membrane separation device while maintaining high pressure energy. A variety of energy recovery apparatuses have been practically applied in order to effectively utilize the high pressure energy possessed by this outflowing condensed seawater.
One example of a conventional energy recovery apparatus in a seawater desalination system of the reverse osmosis method is described with reference to FIG. 8 to FIG. 11G. FIG. 8 is a schematic view which shows the configuration and water flow of a conventional example of an energy recovery apparatus used in a seawater desalination system, and shows a condition where a first cylinder device is in a pressure feeding process, and a second cylinder device is in the filling process. FIG. 9 is a schematic view of FIG. 8, and shows a condition where the first cylinder device is in the filling process, and a second cylinder device is in the pressure feeding process. FIG. 10 is a diagram which shows the structure of a conventional flow path switching device of FIG. 8. FIGS. 11A-11G are schematic views which show operational relationships between the flow path switching device and the pressure feeding process and filling process of the conventional first and second cylinder devices of FIG. 8.
In FIG. 8, the pressure of seawater that is taken in by a water intake pump 1 serving as the water supply means is increased by a high-pressure pump 2 (to 5-7 MPa in this example), and is sent to a membrane separation device 3 to be separated into condensed seawater 4 and fresh water 5. The separated fresh water 5 runs into a storage tank (not illustrated). On the other hand, the high-pressure condensed seawater 4 which flows out from an outlet 23 of the membrane separation device 3 flows into a flow path switching device 6 from an intake port 6a, flows into the other end of a first cylinder device 7a connected to a first inflow/outflow port 6b of the flow path switching device 6, and moves a piston 8a under high pressure in the direction of the arrow marks. The low-pressure seawater inside the first cylinder device 7a is compressed by the movement of the piston 8a, raising its pressure. This seawater flows out from a first communication port 9 at one end, and flows into a booster pump 11 that serves as a pressure boosting means via a flow path direction regulation device 10 configured with four check valves, where it is raised in pressure to the same discharge pressure as the high-pressure pump 2, after which it is merged with high-pressure seawater from the high-pressure pump 2, fed to the membrane separation device 3, and separated into high-pressure condensed seawater 4 and fresh water 5.
The filling process in a second cylinder device 7b is conducted in parallel with the aforementioned pressure feeding process of the first cylinder device 7a. The seawater taken in by the water intake pump 1 is supplied to one end of the second cylinder device 7b from a second communication port 12 via the flow path direction regulation device 10, and moves a piston 8b in the direction of the arrow marks. The low-pressure condensed seawater in the second cylinder device 7b is pushed out by the movement of the piston 8b, flows into the flow path switching device 6 from a second inflow/outflow port 6c of the flow path switching device 6, and is discharged from a second outflow port 6e to a drainage path 60. The piston 8a of the first cylinder device 7a in the pressure feeding process moves to the flow path direction regulation device 10 side, and when the piston 8a is detected by a first position detector 13a provided at the one end side, a first signal is transmitted from the first position detector 13a to a controller 14. Upon receiving this first signal, the controller 14 sends a switching signal to a drive device 15 of the flow path switching device 6, whereupon the flow path of the flow path switching device 6 is switched.
As a result of the flow path switching of this flow path switching device 6, the second cylinder device 7b is switched to the pressure feeding process, and the first cylinder device 7a is switched to the filling process, as shown in FIG. 9. The high-pressure condensed seawater 4 which flows out from the outlet 23 of the membrane separation device 3 flows into the other end of the second cylinder device 7b which communicates with the second inflow/outflow port 6c of the flow path switching device 6, and moves the piston 8b under high pressure in the direction of the arrow marks. The low-pressure seawater in the second cylinder device 7b is compressed by the movement of the piston 8b, raising its pressure. This seawater flows out from one end from the second communication port 12, flows into the booster pump 11 via the flow path direction regulation device 10, merges with high-pressure seawater from the high-pressure pump 2 under the same discharge pressure as the high-pressure pump 2, and runs into the membrane separation device 3.
The filling process in the first cylinder device 7a is conducted in parallel with the pressure feeding process of this second cylinder device 7b. The seawater taken in by the water intake pump 1 is supplied to one end of the first cylinder device 7a from the first communication port 9 via the flow path direction regulation device 10, and moves the piston 8a in the direction of the arrow marks. The low-pressure condensed seawater in the first cylinder device 7a is pushed out by the movement of the piston 8a, flows into the flow path switching device 6 from the first inflow/outflow port 6b of the flow path switching device 6, and is discharged from the first outflow port 6d to the drainage path 60. The piston 8b of the second cylinder device 7b in the pressure feeding process moves to the flow path direction regulation device 10 side, and when the piston 8b is detected by a second position detector 13b provided at the one end side, a second signal is transmitted from the second position detector 13b to the controller 14. Upon receiving this second signal, the controller 14 sends a switching signal to the drive device 15 of the flow path switching device 6, whereupon the flow path of the flow path switching device 6 is switched.
By alternately switching the first cylinder device 7a and the second cylinder device 7b between the pressure feeding process and the filling process in this manner, and by continuously conducting this alternate switching, energy recovery can be effectively conducted. The pertinent energy recovery apparatus is disclosed in U.S. Pat. No. 5,797,429.
As shown in FIG. 10, with respect to the conventional flow path switching device 6 used in FIG. 8 and FIG. 9, there is sequential disposition of the second outflow port 6e, second inflow/outflow port 6c, intake port 6a, first inflow/outflow port 6b, and first outflow port 6d in a single cylinder 16. The first outflow port 6d and the second outflow port 6e communicate with the drainage path 60, the intake port 6a communicates with the outlet 23 of the membrane separation device 3, the first inflow/outflow port 6b communicates with the other end of the first cylinder device 7a, and the second inflow/outflow port 6c communicates with the other end of the second cylinder device 7b. Furthermore, there is provision of a first piston 17a which causes blockage and communication between the first outflow port 6d and the first inflow/outflow port 6b as well as blockage and communication between the first inflow/outflow port 6b and the intake port 6a, and a second piston 17b which causes blockage and communication between the second outflow port 6e and the second inflow/outflow port 6c as well as blockage and communication between the second inflow/outflow port 6c and the intake port Ga. This first piston 17a and second piston 17b are connected by a piston rod 18, and the piston rod 18 is configured to connect to the drive device 15.
The operations of the conventional flow path switching device 6 of the pertinent configuration are described with reference to FIGS. 11A-11G. With respect to FIG. 11A, the first cylinder device 7a is in the pressure feeding process, and the second cylinder device 7b is in the filling process. The first piston 17a blocks communication of the first inflow/outflow port 6b and the first outflow port 6d, the intake port 6a communicates with the first inflow/outflow port 6b, and the high-pressure condensed seawater 4 flows into the first cylinder device 7a. The second piston 17b blocks communication of the intake port 6a and the second inflow/outflow port 6c, the second inflow/outflow port 6c communicates with the second outflow port 6e, and the low-pressure seawater in the second cylinder device 7b drains into the drainage path 60. FIG. 11B shows the condition where the piston 8a is detected by the first position detector 13a of the first cylinder device 7a, the piston rod 18 moves leftward from the right side of the page so that the flow path switching device 6 switches, and the second inflow/outflow port 6c is obstructed by the second piston 17b. In FIG. 11C, the piston rod 18 moves further, the second inflow/outflow port 6c is completely obstructed by the second piston 17b, and the second inflow/outflow port 6c and the second outflow port 6e are blocked while the second inflow/outflow port 6c and the intake port 6a remain blocked. In this state, the first inflow/outflow port 6b is not yet obstructed by the first piston 17a. Now, when the first inflow/outflow port 6b is obstructed by the first piston 17a in a state where the second inflow/outflow port 6c is blocked by the second piston 17b, the inflow of high-pressure condensed seawater 4 into the intake port 6a is interrupted, and a high-pressure load is exerted upon the membrane separation device 3, which entails the risk of damage to the reverse osmosis membrane, and occurrence of vibration and noise in the flow path switching device 6 and its piping, and which also results in a decline in energy recovery efficiency. Then, as in FIG. 11D, the first inflow/outflow port 6b starts to be obstructed by the first piston 17a, and to the extent that the flow rate of inflowing high-pressure condensed seawater 4 is reduced by contraction of flow path area, obstruction of the second inflow/outflow port 6c by the second piston 17b is cleared with expansion of flow path area, and the flow rate of high-pressure condensed seawater 4 flowing into the second inflow/outflow port 6c increases. By this means, without hindering the flow of high-pressure condensed seawater 4 that flows into the intake port 6a, there is prevention of damage to the reverse osmosis membrane, and occurrence of vibration and noise in the flow path switching device 6 and its piping, and there is also improvement in energy recovery efficiency. When, as a result of further movement of the piston rod 18 as in FIG. 11E, the first inflow/outflow port 6b is completely obstructed by the first piston 17a, blocking communication between the first inflow/outflow port 6b and the intake port 6a, the second inflow/outflow port 6c and the intake port 6a come into full communication by displacement of the second piston 17b. As a result of further movement of the piston rod 18 as in FIG. 11F, obstruction of the first inflow/outflow port 6b by the first piston 17a is cleared, and the first inflow/outflow port 6b and the first outflow port 6d come into communication in a small flow path area, and as a result of still further movement of the piston rod 18 as in FIG. 11G, obstruction of the first inflow/outflow port 6b by the first piston 17a is completely cleared, and the first inflow/outflow port 6b and the first outflow port 6d communicate. By the switching of the flow path switching device 6 which moves the piston rod 18 rightward from the left side of the page by a first signal from this first position detector 13a, the first cylinder device 7a now enters the filling process, and the second cylinder device 7b enters the pressure feeding process. By the switching of the flow path switching device 6 which moves the piston rod 18 leftward from the right side of the page by a second signal from the second position detector 13b, the first cylinder device 7a now again enters the pressure feeding process, and the second cylinder device 7b enters the filling process.
With respect to the flow path switching device 6 in the above-described conventional energy recovery apparatus, as shown in FIG. 10, if the width of the intake port 6a is X1, the widths of the first inflow/outflow port 6b and second inflow/outflow port 6c are identical at X2, the widths of the first piston 17a and second piston 17b are identical at X3, and the width of the first outflow port 6d is X4, in order for the first piston 17a and second piston 17b to move from the condition indicated by the solid lines to the condition indicated by the broken lines, it is necessary that the cylinder 16 have the length of X1+2×X2+6×X3+X4. The width X3 of the first piston 17a and second piston 17b is set larger than the width X2 of the first inflow/outflow port 6b and second inflow/outflow port 6c. This is in order that there will be complete obstruction when the first inflow/outflow port 6b and second inflow/outflow port 6c are respectively obstructed by the first piston 17a and second piston 17b. If interstices occur without complete obstruction, there is a risk that the high-pressure condensed seawater 4 of the intake port 6a would flow to the first outflow port 6d and second outflow port 6e. The stroke of the piston rod 18 requires only X2+2×X3. The width of the first inflow/outflow port 6b and the width of the second inflow/outflow port 6c are identically set to X2, and the width of the first piston 17a and the width of the second piston 17b are identically set to X3 so that high-pressure condensed seawater 4 will similarly flow into the first cylinder device 7a and second cylinder device 7b. 
The above-described conventional energy recovery apparatus has the feature that energy exchange can be continuously conducted, because high-pressure condensed seawater 4 continuously flows into the intake port 6a at a constant fixed flow rate without interruption, and the first inflow/outflow port 6b that communicates with the first cylinder device 7a and the second inflow/outflow port 6c that communicates with the second cylinder device 7b are not simultaneously obstructed. However, in the transition period of flow path changeover of the flow path switching device 6, there is abrupt fluctuation in the flow rate and pressure of the high-pressure condensed seawater 4, vibration and noise occurs, and the reverse osmosis membrane may be damaged due to such factors as the change in flow path and the responsiveness of the check valves used in the flow path direction regulation means 10. The efficiency of energy recovery also declines.
As it is necessary to provide five ports in a cylinder 16 of a single flow path switching device 6, the cylinder 16 and the piston rod 18 lengthen in the axial direction, and it is difficult to machine with satisfactory precision the inner diameter of the cylinder 16, and the outer diameter of the first piston 17a and second piston 17b fixed to the piston rod 18. Furthermore, due to the lengthened stroke of the piston rod 18 of the flow path switching device 6, the drive device 15 (using, for example, a hydraulic cylinder or the like) which drives the piston rod 18 is enlarged. Moreover, as the cylinder 16 is long in the axial direction, it is necessary to accelerate the travel speed of the first piston 17a and the second piston 17b, and accelerate flow path switching speed, but pressure fluctuations within the flow path switching device 6 are increased by acceleration of switching speed, giving rise to vibration and noise, and shortening the life of the flow path switching device 6, with the additional result that the size of the overall energy recovery apparatus increases, and a large installation space is required